Problem with lukewarm temp on Pegasus 1/4 turn 1 handle shower/tub

We redid the bathroom in our money pit (also known as the foreclosure that we should have not bid on) and the "handyman" that installed the pegasus 9000 Brushed Nickel Single Handle Tub and Shower (Model 873-0004) threw away the instructions which is now posing a problem possibly. When I shower the water is just lukewarm. If I turn off the cold shut off valve, the hot just trickles out of the bath spout. But when I turn on the faucet to take a shower the hot water intake pipe is raging hot, even past the shut off valve to the mixer, so I can presume the valve isn't stuck is a semi closed position. Is it possible there is some kind of safety switch or something? I am freezing here!!

Most (all?) new shower valves have an adjustment that limits how much hot water can mix into the output. How that is adjusted on this faucet, I do not know. But, if you take the handle and maybe the trim off, it may become evident. There'll be a screw, cam, lever, or something that you can adjust to increase the amount of hot. I've seen a couple of valves that had it set to max hot out of the box, yours apparantly is set to a much milder setting out of the box (or adjusted by the installer). Some people adjust them summer and winter to account for the colder incoming water temps. But if you adjust it now, you may find max it too warm in the summer.

All new shower valves are required to have anti-scald technology, so if you turn off the cold, it should actually stop the hot...I suppose a small trickle might still qualify. Be aware, though, that sometimes this will stick if you don't have both the hot and cold supplies on. Often, you can just tap it to get it free, but it gets worse sometimes as they get older and the seals get stiffer or they get mineral deposits on them.